The aim of work presented in this thesis was to explore the inter-relationships of cyclic sedimentation, relative sea-level change, and palaeoclimate as inferred from plant megafossils. To this end, the investigation focused on the classic plant-bearing Middle Jurassic succession of Yorkshire. The Middle Jurassic (Aalenian-Bathonian) Ravenscar Group of the Cleveland Basin (Yorkshire) comprises a predominantly fluvio-deltaic succession intercalated between thinner, laterally persistent marine units. There is a pronounced lateral facies change across the basin, from mainly alluvial sediments in the north to more marine deposits in the south. Although variable in character, the facies composing the sequence are described by four principal environments of deposition: alluvial, estuarine, lagoonal and marine. In an attempt to achieve a more accurate stratigraphic control on the succession, sequence-stratigraphic concepts are applied to outcrop exposures and subsurface cores. The identification of 'key surfaces' in the sequence resolves a series of lithological cycles which reflect relative sea-level fluctuations. Using this approach the Aalenian-Bathonian sequence can be subdivided into two large-scale (second-order) transgressive-regressive cycles onto which six medium-scale (thirdorder) cycles of transgression and regression are superimposed. The potential for correlating these lithological cycles regionally has been assessed by comparing coeval sections from southern Scandinavia. Plant-bearing fluvio-deltaic sequences from Bornholm and Scania were chosen as a means for appraising the lateral continuity of the cycles, and assessing what factors might have controlled their development. Study of floral remains from the Ravenscar Group within the context of this stratigraphic framework yields valuable palaeoclimatic information. Growth-ring analysis of fossil wood of Late Pliensbachian to Late Bathonian age indicates a distinctly seasonal climate with low to moderate interseasonal variation in tree growth. Significant intraseasonal influences on wood production are implicit in the abundance of false rings. Consideration of these results within a stratigraphic context suggests that conditions during the Bathonian were comparatively hostile: a finding which is interpreted to be due to more frequent and extended water shortages associated with a drier climate. These palaeoclimatic inferences are substantiated by evidence obtained from the examination of the flora using Correspondence Analysis (CA). This approach verifies the presence of a temporal fluctuation in the flora found by previous investigations and, furthermore, highlights physiognomic trends in the flora with time. The results from CA also indicate adverse growing conditions during the Bathonian, emphasized by the prevalence of xeromorphic taxa.

Dinoflagellates are a diverse group of flagellated eukaryotes, the majority of which are marine plankton, and are one of the most important groups of primary producers in the world’s oceans. The dinoflagellate fossil record is based on their zygotic resting cysts; this record indicates that the Bajocian of the Mid Jurassic (~170–168 Ma) represents a critical interval in dinoflagellate evolutionary history, marked by a rapid increase in the diversity of cysts from the family Gonyaulacaceae. From the Bajocian onwards, the Gonyaulacaceae have remained one of the most diverse and abundant groups of dinoflagellate cysts in the fossil record. Even so, Bajocian dinoflagellate cysts themselves have received relatively little study, leaving the patterns of this radiation unresolved. In this thesis, I examine the Bajocian diversification of gonyaulacacean dinoflagellate cysts in northwest Europe via quantitative palynological analysis, and relate this into a broader stratigraphic and palaeoenvironmental context. The dinoflagellate cyst record of the three key study areas demonstrates an increase in gonyaulacacean dinoflagellate cyst diversity through the Bajocian, irrespective of differing palaeoenvironmental settings. However, palynological and sedimentological data record systematic changes in lithostratigraphic composition and/or depositional environment which reflect changes in sea level. The integration of these data with biostratigraphic records indicates that the pattern of the radiation in Europe was strongly influenced by sea level, with the increase in gonyaulacacean diversity mirroring a major second-order transgression. On a finer scale, the main pulses of first appearances correlate with third-order transgressive episodes. A rise in sea level, coupled with changes in the tectonic configuration of ocean gateways, appears to have controlled the pattern of diversification in Europe. These palaeoceanographic changes may have enhanced water-mass transfer between Europe, the northwest Tethys Ocean, and the Hispanic Corridor, which promoted the floral interchange of dinoflagellates. Comparison to global data demonstrates that gonyaulacacean dinoflagellate cysts increased in diversity on a global scale through the Mid Jurassic. Whilst sea level rise and associated large-scale palaeoenvironmental shifts appear to have controlled the pattern of dinoflagellate cyst appearances in several regions (e.g. eastern Canada, Israel), there is no direct correlation between dinoflagellate cyst diversity and sea level rise on a global scale. Within dinoflagellate cyst assemblages from Europe, the spatial and temporal distribution of key taxa can be related to differing palaeoenvironmental settings of the basins studied; but certain patterns may reflect wider palaeoenvironmental drivers. Carbon isotope records generated for this thesis, and their correlation to other European sections, demonstrate that the Early Bajocian in Europe was marked by a positive shift in δ13C. Previous work has linked this carbon cycle perturbation to a phase of enhanced continental weathering and associated run-off. In several European basins, the Early Bajocian was marked by an acme of the genus Dissiliodinium; this genus may have bloomed in response to elevated nutrient levels. Intriguingly, a similar pattern is seen within dinoflagellate cyst assemblages from Australia, indicating this interval of palaeoenvironmental change may have had a global extent.

No description available.

The exine ultrastructure of Williamsoniella coronata Thomas from the Bajocian of Yorkshire (United Kingdom) was investigated with light, scanning electron, and transmission electron microscopy. The pollen averages 16.5 μm alongitsshort axisand24.5μmalongitslongaxis andismonosulcate, and thenonapertural sculpturingisdistinctly verrucate. The pollen wallishomogeneous, and the sulcus membraneiscomposedofthin exine withscattered small granules. The pollen grains differ in exine sculpturing and pollen wall ultrastructure from pollen grains of the bennettitalean taxa Cycadeoidea dacotensis (MacBride) Ward and Leguminanthus siliquosis (Leuthardt) Kraeusel. They are similar todispersed pollen grainsof Granamonocolpites luisae Herbst from the Triassic Chinle Formation of the United States, supporting the bennettitalean affinity of these dispersed pollen grains. The Bennettitales are palynologically characterized by monosulcate "boat-shaped" pollen with a homogeneous or granular pollen wall ultrastructure.

No description available.

Thirty cores from the Late Jurassic uppermost Jubaila, Arab-D reservoir and Arab-D anhydrite in Hawiyah (Ghawar) and Harmaliyah fields, eastern Saudi Arabia were studied to document the detailed facies stacking and high resolution sequence stratigraphy. The Jubaila-Arab-D interval is a shallowing upward succession of two composite sequences, in which the Arab-D reservoir and overlying anhydrite have up to twelve higher frequency sequences. Both fields are strikingly similar in terms of facies, parasequences, and vertical stacking of facies. The direction of the progrodaion is east and northeast and that is supported by northeast thickening of the Arab-D reservoir and by the stromatoporoid and Cladocoropsis facies progrodation. This suggests that the Arab-D reservoirs in both fields may represent part of a single carbonate ramp with subtle syndepositional highs. The scarcity of exposure surfaces with caliche in the Arab-D reflects the relatively high subsidence rate (~6 cm/k.y.) relative to the small sea level oscillations that formed the succession coupled with the long term shallowing trend up through the section. Dolomites from the Arab-D reservoir zones 1 to 4, in both fields were studied to better understand their origin. The dolomites are dominantly fabric destructive medium to coarse grained types, and much less common fabric retentive finer grained dolomites in the uppermost Arab-D reservoir. The δ¹³C values are rock buffered while the δ¹⁸O values have been greatly shifted toward negative values relative to unaltered early dolomite, and dolomite crystal rims generally have lighter δ¹⁸O values than cores. The dolomites were initiated at different times during shallowing phases on the Arab-D platform, with the bulk of the fabric destructive dolomites forming under near normal salinities, while the fabric preserving dolomites formed as a result of dolomitizing aragonitic sediments from more evaporated waters. With increasing burial and increasing temperature, the early dolomites re-equilibrated with the increasingly warm basinal brines resulting in replacement of cores, and dolomite cementation by rim overgrowth. Progressive plugging of higher dolomites earlier, caused some of these to retain slightly heavier δ¹⁸O values and marine seawater Sr isotope values while those that remained permeable developed very light δ¹⁸O values and more radiogenic Sr values, shifting them toward the field of late stage baroque dolomite. / Ph. D.

No description available.

Jurassic volcanic rocks in southeastern Arizona provide an opportunity to study the paleomagnetism of an autochthonous segment of the Mesozoic Cordilleran magmatic arc. The Corral Canyon sequence in the Patagonia Mountains is a 650 meter thick homoclinal sequence consisting of interbedded volcaniclastic red-beds, welded ash-flow tuff, and lavas. Rb/Sr isotopic analysis of eight whole rock tuff samples yields an isochron age of 171 ± 3 Ma. Welded tuffs in the Corral Canyon sequence possess a stable, primary magnetization carried in both magnetite and hematite that defines a paleomagnetic pole at 61.8°N, 116.0°E, alpha₉₅= 6.2°. This pole is considered to be a reliable Middle Jurassic reference pole for cratonic North America. Paleomagnetic study of the Canelo Hills volcanics welded tuff member also yields a stable, primary magnetization throughout a stratigraphic thickness of 600 meters. However, results from this formation are enigmatic and the mean pole is discordant with respect to Middle Jurassic reference poles. Various aspects of the paleomagnetic data indicate that discordance of the Canelo Hills volcanics pole is probably due to acquisition of remanent magnetization during a period of non-dipole behavior of the geomagnetic field. Dispersion of paleomagnetic directions suggests that the welded tuff member represents at most two cooling units and can be interpreted as a caldera-fill sequence. A revised Jurassic APW path differs significantly from available paths and has important implications for North American plate motion and paleolatitude. The spatio-temporal progression of reliable Jurassic paleopoles, in conjunction with Triassic and Early Cretaceous poles, is well described by paleomagnetic Euler pole analysis. The APW path is divided into three tracks, separated by two cusps. These cusps represent changes in the direction of North American absolute plate motion and can be correlated with global plate motion and intraplate deformation events at approximately 200-210 Ma and 150 Ma. Finally, the APW path presented herein predicts more southerly Late Triassic and Jurassic paleolatitudes for North America than have been suggested by previous authors. Using revised reference poles, there are no inclination anomalies within paleomagnetic data from Late Triassic and Early Jurassic rocks of Stikinia and Quesnellia (B.C., Canada).

This study utilised U-Pb geochronology, and 40Ar/39Ar and (U-Th)/He thermochro-nology to delineate arc magmatism, metamorphism, and exhumation of the pericratonic Yukon-Tanana terrane in the McQuesten map area of west-central Yukon, Canada. SHRIMP U-Pb ages delineate Mid to Late Paleozoic arc magmatism and fit key units into the regional lithotectonic framework of the terrane. The juxtaposition of unmetamorphosed and predomi-nantly undeformed Devono-Mississippian rocks in the northwest of the study area with polydeformed and up to amphibolite facies metamorphosed rocks in the southwest suggests a crustal-scale discontinuity, the Willow Lake fault, bounds the two domains. The asymmetric distribution of 40Ar/39Ar ages across the fault suggest it is extensional, and was active in the Early Jurassic. Zircon (U-Th)/He ages delineate erosion of rocks in the northwest through the upper crust during the Late Triassic and Late Jurassic to Early Cretaceous followed by Mid-dle Cretaceous erosion of the southwestern domain and possibly fault reactivation.

The latitudinal motion of the Tibetan Himalayathe northernmost continental unit of the Indian plateis a key component in testing paleogeographic reconstructions of the Indian plate before the India-Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have wasp-waisted hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency-dependent magnetic susceptibility. Volcaniclastic rocks exhibit pot-bellied hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine-grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.